A process is known for preparing actinium-235 from thorium-229 and daughter fission products, this process comprising the dissolution of a sample in a nitric acid solution and the ion-exchange recovery of actinium-235 from parent thorium-229 [RU No. 2200581].
A drawback of this process is a limited availability of the raw material (thorium-229), which is in turn produced from uranium-233. Therefore, potential productions are not high.
Another process for producing actinium-235 comprises the irradiation of targets made of radium salts with protons in a cyclotron followed by ion-exchange separation of actinium and radium [U.S. Pat. No. 6,299,666 B1].
A drawback of this process consists of the hazard of radium salts. Further, they have high thermal conductivities and thereby cannot be irradiated with high currents. Furthermore, these targets have high costs, and radium regeneration is thereby necessary.
One more process for producing actinium-235 comprises the irradiation of targets containing metallic thorium with proton beams having energies higher than 40 MeV, dissolution of irradiated thorium in nitric acid, and subsequent recovery of actinium-235 from the solution. Thorium and newly formed protactinium were separated front actinium and radium by means of precipitation in the form of iodates, and actinium was separated from radium by extraction with thenoyl trifluoroacetate [see H. Gauvin, Reactions (p, 2pxn) sur le thorium 232 de 30 a 120 MeV, Journal de Physique, Vol. 24, pp. 836-838, 1963]. This process fails to provide the recovery of actinium from thorium targets of large weights and targets containing large amounts of isotopes of other elements generated by proton bombardment.
The most pertinent art consists of the process for producing actinium-235, this process comprising the irradiation of targets containing metallic thorium in the form of a foil with protons in a cyclotron, the dissolution of targets in a nitric acid solution, and recovery of actinium [see M. Lefort et al., Reactions nucleaires de spallation induites sur le thorium par des protons de 150 et 82 MeV, Nuclear Physics, Vol. 25, pp. 216-247, 1961].
A drawback of this process consists of small weights of the thorium targets used (foil thicknesses are up to 0.05 mm), which cannot provide high yields of actinium. Chemical recovery methods are practically unsuitable for processing high-activity thorium targets of great weights for producing large amounts of 225Ac. Further, the process does not provide refining of actinium from a number of foreign isotopes which are generated in large amounts in a proton-irradiated thorium target, and thereby cannot provide a high purity of the final products.
A process for producing radium isotopes comprises chemical recovery from a small weight amount of 227Th (having a half-life period of 18.7 days), which is in turn produced by decay of 235/(7×108 years)→231Pa (32,800 years) 227Ac (28 years) [see G. Henriksen et al., 223Ra for Endoradiotherapeutic Applications Prepared from Immobilized 227Ac/227Th Source, Radiochim. Acta, Vol. 89, pp. 661-666, 2001].
A drawback of this process consists of the following: the amount of 227Ac that can be recovered from natural uranium-235 is small; in producing 227Ac by irradiation of a 226Ra target in a nuclear reactor, the target is dangerous to handle, has a high cost, and is not easily accessible, thereby requiring radium regeneration after irradiation and refining from numerous radioactive fission products.
Another process for producing radium isotopes comprises the irradiation of targets containing metallic thorium in the form of a foil with protons in a cyclotron, the dissolution of targets in a nitric acid solution, and recovery of radium [see. M. Lefort et al., Reactions nucleaires de spallation induites sur in thorium par des protons de 150 et 82 MeV, Nuclear Physics, Vol. 25, pp. 216-247, 1961].
A drawback of this process also consists of small weights of the thorium targets used (foil thicknesses are up to 0.05 mm), which cannot provide high yields of radium. Chemical recovery methods are also practically unsuitable for processing high-activity thorium targets of great weights for producing large amounts of radium isotopes (223Ra, 225Ra, and 224Ra). Further, the process does not provide refining of radium from some foreign isotopes which are generated in large amounts in a proton-irradiated thorium target, and cannot provide a high purity of the final products.
The most pertinent art is the process for producing radium isotopes, comprising the irradiation of thorium metal containing targets with beams of accelerated charged particles [see L. N. Moskvin and L. G. Tsaritsyna, Recovery of Actinium and Radium from a Thorium Target Irradiated with 660-MeV Protons, At. En., Vol. 24, pp. 383-384, 1968]. In order to recover radium isotopes, a thorium target was first dissolved in nitric acid, and the solution provided by thorium dissolution was admitted to a column packed with a sorbent coated with tributyl phosphate. Thorium, protactinium, zirconium, hafnium, and niobium were retained in the column, whereas actinium, radium, alkali elements, and alkaline-earth elements passed through it. Additional separation of radium from actinium, together with other alkaline-earth elements, was performed on a column packed with a sorbent coated with di-2-ethylhexylphosphoric acid.
A drawback of this process consists of the following: with use of bulky thorium targets in producing considerable amounts of radium, the precipitation of thorium will require very large columns. Further, the process does not provide the purification of radium from other alkaline-earth elements and from other fission products.
A target is known for use in the production of Rn, Xe, At, and I radioisotopes, this target comprising a thorium-238 sample to be irradiated wrapped in an aluminum foil [see U.S. Pat. No. 4,664,869].
A drawback of this target consists of small weights of the thorium targets used (of about 1 g), which cannot provide high activity yields of actinium and radium.
Another target is known for use in the production of actinium-235 and radium isotopes comprising a thorium metal target designed as a foil [see M. Lefort et al., Reactions nucleaires de spallation induites sur le thorium par des protons de 150 et 82 MeV, Nuclear Physics, Vol. 25, pp. 216-247, 1961].
A drawback of this target consists of small weights of the thorium targets used (foil thicknesses are up to 0.05 mm), which cannot provide high activity yields of actinium and radium.
Targets made of radioactive materials which are cooled while being irradiated in an accelerator or a reactor are, as a rule, enclosed in air-tight shells.
The most pertinent art consists of a target that comprises a thorium metal sample to be irradiated enclosed in an air-tight shell which is cooled with a liquid during irradiation [see US 2006/0072698, 2006].
A drawback of this target consists of the following: it is purposed for being irradiated with low-energy protons (below 40 MeV) and should have a relatively small thickness, and the material of the target shell (aluminum or silver) can melt or degrade when exposed to an intense beam of charged particles on account of interaction with thorium or a cooling liquid agent (aluminum); further, the target and shell thicknesses are not defined and it is not specified how the shell can be made air tight. Furthermore, real experimental data are not given in this reference.
The objects of the present invention are to solve the aforementioned problems by means of irradiating a thick (up to several centimeters) target of thorium metal with a high (tens of microampers) current of a charged particle beam and to separate pure actinium and radium from thorium and generated radioactive isotopes of various elements, such as protactinium, cesium, strontium, lanthanum, barium, zirconium, niobium, iodine, ruthenium, rhodium, antimony, and others. The technical result provided by the invention consists of an enhancement of yields of actinium-235, radium-223, and other radium isotopes.